Transcript Acids and Bases and Aqueous Equilibria

History on PowerPoint:
Acids and Bases
David A. Katz
Department of Chemistry
Pima Community College – West
Campus
2202 W. Anklam Rd
Tucson, AZ 85709 USA
Email: [email protected]
The Arrhenius Theory
• Svante August Arrhenius (1859 –1927)
– Acid: Substance that produces
hydrogen ions in water solution.
HCl(aq) → H+(aq) + Cl- (aq)
– Base: Substance that produces
hydroxide ions in water solution.
NaOH (aq) → Na+(aq) + OH-(aq)
– An acid neutralizes a base
H+(aq) + OH-(aq) → H2O(l)
The Brønsted–Lowry Theory
Johannes Nicolaus Brønsted (1879-1947)
and Thomas Martin Lowry (1874-1936)
independently proposed the proton
definition of acids and bases in 1923
Quoting Brønsted:
". . . acids and bases are substances that are
capable of splitting off or taking up hydrogen
ions, respectively.“
or
An acid-base reaction consists of the transfer of
a proton (or hydrogen ion) from an acid to a
base
The Brønsted–Lowry Theory
An acid is a proton donor
That is, an acid is a substance from which a
proton can be removed.
A Base is a proton acceptor
That is, a base is a substance that has a nonbonded electron pair that can bond with a
proton from an acid.
No solvent is specified
The Lewis Theory
Gilbert Newton Lewis (1875-1946)
In a 1923 paper, Lewis wrote:
"We are so habituated to the use of water as a
solvent, and our data are so frequently limited
to those obtained in aqueous solutions, that
we frequently define an acid or a base as a
substance whose aqueous solution gives,
respectively, a higher concentration of
hydrogen ion or of hydroxide ion than that
furnished by pure water. This is a very one
sided definition . . . .“
The Lewis Theory
Lewis wanted a general definition of an acid and a base:
"When we discuss aqueous solutions of substances which do not
contain hydroxyl [ion], it is simplest to define a base as a
substance which adds hydrogen ion. . . . Since hydrogen is a
constituent of most of our electrolytic solvents, the definition of
an acid or base as a substance which gives up or takes up
hydrogen ion would be more general than the one we used
before, but it would not be universal."
Lewis then gave his definition of an acid and a base:
"We are inclined to think of substances as possessing acid or basic
properties, without having a particular solvent in mind. It seems
to me that with complete generality we may say that a basic
substance is one which has a lone pair of electrons which may be
used to complete the stable group of another atom, and that an
acid is one which can employ a lone pair from another molecule
in completing the stable group of one of its own atoms."
pH
• The concept of pH was first introduced by Danish chemist
Søren Peder Lauritz Sørensen (1868-1939), the head of the
Carlsberg Laboratory’s Chemical Department, in 1909
• Dr. Sørensen developed the pH scale during his pioneering
research into proteins, amino acids and enzymes - the basis
of today’s protein chemistry in a paper titled “Enzyme
Studies II. The Measurement and Meaning of Hydrogen Ion
Concentration in Enzymatic Processes”:
The value of the hydrogen ion concentration
will accordingly be expressed by the
hydrogen ion based on the normality factor
of the solution used, and this factor will
have the form of a negative power of 10.
Since in the following section I usually refer
to this, I will explain here that I use the name
"hydrogen
ion
exponent"
and
the
designation PH for the numerical value of the
exponents of this power.
Sørensen and the Carlsberg Chemistry Department
pH
• Today, we refer to pH as meaning ‘the power of hydrogen’.
• The pH scale provides a simple and universal measurement of the
amount of hydrogen ions in a solution, which affects its acidity and
how it reacts chemically.
• Each value of pH means the H+ concentration changes by a factor of
10
• As the H+ concentration decreases, the OH- concentration increases
pH 1
strong
acid
pH 7
weak
acid
neutral
pH 14
weak
base
The pH scale according to the late Dr. Hubert Alyea, Princeton University
strong
base
The pH Meter
• First pH meter was constructed in 1934 by Arnold
Beckman (1900-2004).
• A glass pH electrode that had a potential dependent on
activity of H+ ions had been constructed in 1906 by Fritz
Haber and Zygmunt Klemensiewicz, but there were
technical difficulties due to a large internal resistance of
glass electrodes. To obtain reliable results one was
forced to use very sensitive galvanoscope - expensive
and difficult to maintain.
• To overcome the problem Arnold Beckman proposed to
use simple high-gain amplifier made using two vacuum
tubes. Amplified current was much easier to measure
with cheap miliamperometers.
Beckman’s first pH meter, 1934 (middle photo)
The Model G pH meter, the first successful marketed pH
meter, 1936 (bottom photo)